Geological mapping and geophysical surveying on the earth's surface are mature sciences with a history of technology enhancements that improved the fidelity of understanding of the Earth, above and beneath the surface. Yet when conventional techniques are employed in an underground environment, geo-location has proven a challenge that drives concepts of operations to bootstrap techniques to geo-locate instrumentation and geological contacts and can actually limit the effectiveness of employed technologies.
Conventional mapping and survey systems, such as a Global Positioning System (GPS), determine the location of objects using satellite signals. However, a longstanding problem exists with determining location of personnel and equipment within, for example, underground facilities without the use of surveying. To date, this problem has not been resolved because of the difficulty of signaling/communicating between the Earth's surface and underground facilities/caverns/mines and the complexity of electromagnetic propagation within the Earth.
Lower fidelity very low frequency systems are currently in development in Europe to support communications for cave rescue operations. The systems only obtain a shallow depth position when the communication system is used underground. These communications systems are effective up to 600 m and occasionally up to 1,200 m. The systems are also used to locate underground transmitters at comparable depths. In controlled experiments, they have achieved an accuracy of 2% in horizontal position and only 5% in depth.
The typical means of providing time base synchronization between a transmitter and receiver used for navigation purposes has been to either (1) provide a uniform time radio reference signal from an independent source (GPS or VLF signal) or (2) provide each transmitter and receiver with its own highly accurate and stable timing mechanism which are then mutually synchronized at the beginning of the period of interest. In underground environments, GPS and VLF signals are either unavailable or unreliable. Providing each device with its own stable time base may be expensive, cumbersome, and wasteful of limited available power supply.
Normal radio frequency wireless communications to/from a sub-surface receiver by a surface transmitter have been unavailable due to the electrical properties of ground, soil and rock. Communications beyond a depth of 100 meters is particularly difficult. A system that provides wireless contact between subterranean and surface locations will be desirable. Particularly such a system that could provide accurate positioning, detection and communications between the Earth's surface and sub-surface.